Optical profilometers include optical probes for taking point-by-point measurements across a test object. The probes typically include a focusing optic to focus light onto a small spot on a test object. The same focusing optic can be used to collect light reflected from the focus spots for purposes of measurement.
Partial coherence interferometers can be used with optical profilometers to interpret depth changes in the focus spot as changes in the surface height of test objects. The light sources of partial coherence interferometers typically produce spatially coherent beams encompassing a band of wavelengths. Object and reference arms of the interferometer convey different portions of the beam into respective engagements with a test object and a reference object en route to a spectrometer. The optical path length of the object arm, which includes the optical probe, is subject to change with the changes in the surface height of the test object.
The spectrometer separates the interfering beams returning from the test and reference objects into spectral components. The interference phase of each spectral component progressively varies across the band of wavelengths. That is, the modulo 2π number of wavelengths spanning a given optical path length difference between the object and reference arms of the interferometer varies with the wavelength. The rate of change in phase as a function of the change in wavelength, which is referred to as a modulation frequency, is known to be proportional to the optical path length difference. Thus, variations in the surface height of the test object, which change the optical path length of the object arm, can be measured by monitoring variations in the modulation frequency. Increases in the optical path length difference between the object and reference arms are associated with increases in the modulation frequency, which is caused by the multiplication of small differences between adjacent wavelengths by the number of wavelengths spanning the increased distance.
A so-called “null position” exists where the optical path length difference between the object and reference arms is zero. Here, the modulation frequency is also zero, a characteristic exploited by white light interferometers. Equal optical path length differences on either side of the null condition, i.e., where the optical path length of the object arm is either longer than or shorter than the optical path length of the reference arm, produce the same modulation frequencies.
To avoid such ambiguity, partial coherence interferometers can be used with other measuring instruments to either distinguish between measurements taken on opposite sides of the null position or to limit the range of measurement to just one side of the null position. The additional instrumentation adds cost and complexity and can be difficult to implement over the intended range of measurement with the desired accuracy.